For example, Japanese Patent Laying open Publication (Sho) Nos. 58-0206657, 59-135210 and 59-184243 disclose the process for producing a heat-resistant copolymer including N-phenylmaleimide. However, this process may improve the heat resistance of the resin, but the physical properties such as processability and impact resistance are deteriorated.
Further, U.S. Pat. Nos. 3010936 and 4659790 disclose the process for producing a heat-resistant copolymer including .alpha.-methylstyrene.
However, these processes a I so have the drawbacks that convetsion, latex stability, moldability, and heat stability at high temperature are not good.
The heat-resistant copolymers produced by both the above processes have a poor latex stability and a low solid content.